ATtiny85 blinker using watchdog timer interrupt

attiny_wdt_blink.c

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/wdt.h>
#include <avr/sleep.h>
#include <util/delay.h>

// cribbing from http://electronics.stackexchange.com/a/74850

void prevent_wdt_reset() {
  if(MCUSR & _BV(WDRF)){            // If a reset was caused by the Watchdog Timer...
    MCUSR &= ~_BV(WDRF);                 // Clear the WDT reset flag
    WDTCR |= (_BV(WDCE) | _BV(WDE));   // Enable the WD Change Bit
    WDTCR = 0x00;                      // Disable the WDT
  }
}

void setup_wdt() {
  cli(); // with global interrupts disabled,
  // Set up Watch Dog Timer for Inactivity
  WDTCR |= (_BV(WDCE) | _BV(WDE));   // Enable the WD Change Bit
  WDTCR =   _BV(WDIE) |              // Enable WDT Interrupt
    _BV(WDP2) | _BV(WDP1);   // Set Timeout to ~1 seconds (or something)
  sei(); // now re-enable global interrupts
}

#define FLED 0
volatile uint8_t FLAGS = 0x00;

// ISR for watchdog timer just toggles a bit for now
ISR(WDT_vect) {
  FLAGS ^= _BV(FLED);
}

int main(void) {
  // first things first
  prevent_wdt_reset();

  // now re-set up WDT
  setup_wdt();

  DDRB |= 1 << PB0;

  for(;;) {
    // turn off LED
    PORTB &= ~_BV(PB0);
    // now set it to state of led flag
    PORTB |= ((FLAGS >> FLED) & 1) << PB0;
    _delay_ms(10);
  }

  return 0; // never reached
}